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URBANA 


STATE  OF  ILLINOIS 

DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

A.  M.  SHELTON.  Director 

DIVISION   OF   THE 
STATE  GEOLOGICAL  SURVEY 

M.  M.  LEIGHTON.  Chief 


REPORT  OF   INVESTIGATIONS -NO.   14 

PRESENT  STATUS  OF  CORRELATION  OF 
ILLINOIS  COALS 

BY 
HAROLD  E.  CULVER 


PRINTED  BY  AUTHORITY  OF  THE  STATE  OF  ILLINOIS 


URBANA.  ILLINOIS 
1927 


STATE  OF  ILLINOIS 

DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

A.  M.  SHELTON.  Director 

DIVISION  OF  THE 
STATE  GEOLOGICAL  SURVEY 

M.  M.  LEIGHTON.  Chief 


Committee   of  the   Board   of  Natural   Resources 
and   Conservation 

A.   M.   Shelton,   Chairman 

Director  of  Registration   and   Education 

Charles  M.  Thompson 

Representing  the   President  of  the   Uni- 
versity  of   Illinois 

Edson   S.   Bastin 
Geologist 


Jeffersons  Printing  &  Stationery  Co. 
Springfield,  III. 

1927 


PREFACE 

The  paper  herein  published  on  the  "Present  Status  of  Correlation 
of  Illinois  Coals"  is  a  valuable  summary  of  the  concepts  gained  by  the 
author  during-  his  three  years'  study  of  this  phase  of  the  geology  of  the 
Illinois  coal  field  for  this  Survey.  Because  his  work  has  stimulated  a 
more  careful  study  of  coal  correlation  in  Illinois  and  because  he  has 
accepted  a  position  in  another  part  of  the  country  which  prevents  his 
continuing  this  work,  it  seems  advisable  that  this  summary  should  be 
published  and  made  available  for  the  benefit  of  further  workers. 

M.  M.  Leightox,   Chief, 
State    Geological    Survey    Division. 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  Illinois  Urbana-Champaign 


http://archive.org/details/presentstatusofc55714culv 


PRESENT  STATUS  OF  CORRELATION  OF 
ILLINOIS  COALS 

By  Harold  E.  Culver 

INTRODUCTION 

Stratigraphic  studies  of  the  past  two  years  have  revealed  certain 
apparent  discrepancies  in  the  correlation  of  the  Pennsylvanian  beds  of 
the  Illinois  field.  These  had  been  suggested  in  part  by  previous  work 
in  northwestern  Illinois,  where,  as  brought  out  in  an  earlier  paper,1  the 
available  evidence  strongly  suggests  that  the  rocks  formerly  classed  as 
early  Pennsylvanian  are  really  late  Pennsylvanian  in  age. 

The  evidence  as  to  stratigraphic  relations  throughout  the  Illinois 
field  is  as  yet  not  sufficiently  complete  to  establish  a  new  or  even  a 
revised  correlation,  but  in  view  of  the  writer's  departure  from  the  field 
it  seems  desirable  to  indicate  the  nature  and  trend  of  the  evidence  and 
to  suggest  the  general  conceptions  which  have  been  formed  and  which 
should  probably  be  considered  in  any  correlation  statement. 

GENERAL  RELATIONS  OF  THE  ILLINOIS  PENNSYLVANIAN 
The  great  mass  of  rocks  comprising  the  Pennsylvanian  system  in 
the  Illinois  basin  fills  a  very  broad  and  shallow  depression  south  of  the 
latitude  of  Rock  Island.  Although  the  upper  surface  of  the  Pennsyl- 
vanian rocks  is  nearly  flat,  the  lower  surface  slopes  from  the  margin 
toward  the  deepest  part  of  the  basin.  The  mass  as  a  whole  has,  there- 
fore, a  plano-convex  lens  shape  with  the  greatest  thickness  of  the  beds 
where  the  basin  is  deepest.  The  deepest  part  is  not  in  the  center  of  the 
field,  but  is  well  to  the  southeast  in  Wayne  County,  for  the  basin  is  dis- 
tinctly un  symmetrical. 

The  Pennsylvanian  system  is  unevenly  divided  into  three  parts,  a 
thick  Pottsville  formation  below,  a  thinner  Carbondale  in  the  middle, 
and  a  thick  McLeansboro  at  the  top.  The  important  key  horizons  in  the 
section  as  a  whole  are  the  Murphysboro  or  No.  2  coal  and  the  Herrin 
or  No.  6  coal,  which  lie  at  the  base  and  top  respectively  of  the  Carbon- 
dale  formation.  This  division  of  the  Pennsylvanian  system  was  sub- 
stituted for  earlier  divisions  on  the  basis  of  paleobotanical  studies  by 
David  White.2  Enough  evidence  was  found  to  permit  certain  general 
correlations  between   the   Illinois  sections    and    the    type    localities  in 


lCulver,     H.     ]•;..     Pennsvlvanian     correlation     in     northwestern     Illinois:       Geol.     Soc. 
America  Bull.,   vol.   35,   pp.   321-328,   1924. 

-White.  David.  Report  on  field  work  done  in   1907:   Illinois  State  Geol.   Survey  Bull.   8, 
PP.   268-272.    1907. 

Paleobotanical  work   in  Illinois  in   190S:     Illinois  State  Geol.   Survey  Bull.   14,   pp.   293- 
295,  1909 

5 


6  CORRELATION    OF   ILLINOIS   COAI.S 

Pennsylvania.  White  was  satisfied  as  to  the  essential  identity  of  the  I 
coal  horizons  of  the  Northern,  the  Colchester,  and  the  Murphysboro 
fields.  He  was  also  convinced  that  the  so-called  Rock  Island  coal  was 
older  than  these  and  belonged  in  the  I'ottsville  division,  although  dis- 
tinctly younger  than  the  I'ottsville  of  Battery  Rock.  Hardin  County. 
Plants  from  above  the  horizon  of  llerrin.  No.  6,  coal  appeared  referable 
to  a  zone  distinctly  higher,  possibly  Upper  Freeport  in  age. 

It  has  always  been  more  or  less  definitely  recognized  that  the  first 
accepted  limits  of  these  divisions  of  the  Pennsylvanian  were  temporary, 
and  that  certain  features  of  the  division  were  not  strictly  justifiable. 
Such  a  one  is  the  separation  of  the  Herrin  coal  from  its  overlying  lime- 
stone cap  by  the  formation  boundary.  No  widespread  unconformity 
between  the  coal  and  its  cap  rock  was  recognized,  but  the  coal  itself  was 
thought  to  he  one  of  the  readily  recognizable  beds  and  therefore  suited  I 
for  use  as  a  marker  for  the  top  of  the  Carbondale.  It  was  evident  also 
that  there  was  no  prominent  unconformity  below  the  Murphysboro  coal 
to  mark  the  beginning  of  Carbondale  deposition,  hut  that  the  ease  of 
recognition  made  the  use  of  the  Murphysboro  bed  desirable.  It  should 
be  emphasized,  therefore,  that  the  splitting  of  the  Pennsylvanian  was 
based  upon  expediency  in  practical  application  and  was  never  intended 
to  be  final. 

Areally,  the  three  subdivisions  of  the  Pennsylvanian  system  have 
been  considered  to  lie  in  roughly  concentric  bands,  the  lowest  at  the 
outside,  and  the  highest  in  the  central  part  of  the  basin.  This  arrange- 
ment leads  to  the  structural  conception  that  the  beds  occupy  a  broad, 
shallow  trough. 

SOME  APPARENT  DISCREPANCIES 
With  the  foregoing  statement  of  general  relations  in  mind,  it  is  now 
in  order  to  inspect  some  of  the  features  of  the  rocks  of  the  region  which 
appear  to  be  inharmonious  with  the  general  conceptions  outlined.  In 
northwestern  Illinois,  as  already  noted,3  the  apparent  continuity  of  a 
recognizable  lithologic  sequence  from  the  Rock  Island  field  eastward  to 
the  accepted  McLeansboro  area  of  Bureau  County,  strongly  suggests 
that  the  marginal  beds  of  the  Rock  Island  held  are  McLeansboro  rather 
than  I'ottsville  and  indicates  an  overlap  of  considerable  extent  in  late 
Pennsylvanian  time.  On  this  basis,  it  should  he  noted,  the  massive 
sandstone  in  the  vicinity  of  Muscatine.  Iowa,  which  has  been  considered 
as  typical  I'ottsville,  is  in  reality  late  Pennsylvanian. 

\lon<r  the  western  margin  of  the  basin,  notably  in  "Warren.  Schuvler. 
Adams,   and    Brown    counties,   relations    similar    to     those   of    the    Ruck 


:;  Culver,    H.    E.,    op.    cit. 


CORRELATION    OF    ILLINOIS    COALS  7 

Island  region  appear  to  obtain.  Quadrangle  studies  carried  on  under  the 
auspices  of  the  Illinois  Geological  Survey  during  1924  and  1925  have 
established  rather  definitely  that  late  Pennsylvanian  strata  extend  nearly, 
if  not  quite,  to  the  margin  of  the  Illinois  basin  here.  An  interesting 
feature  of  the  Pennsylvanian  section  in  this  part  of  the  basin  is  that  not 
only  are  the  lower  McLeansboro  beds  present,  but  much  later  beds  as 
well.  The  conglomeratic  limestone  of  the  upper  part  of  the  section 
carries  fragments  of  the  basal  McLeanshoro  limestone. 

In  parts  of  Madison  County  there  is  some  reasonable  question  as 
to  the  identification  of  No.  6  coal.  Core  drilling  has  shown  the 
presence  of  other  coal  beds,  at  about  the  same  horizon,  having  similar 
limestone  cap  rocks  and  at  least  locally  developed  median  shale  partings. 

In  Randolph  County,  west  of  Sparta,  a  series  of  shales  and  sand- 
stones which  have  been  considered  to  be  lower  Pennsylvanian  do  not  ap- 
pear to  have  the  stratigraphic  relations  of  such  beds  and  may  be  found 
to  be  stratigraphically  above  No.  6  coal.  If  this  is  the  true  relation, 
upper  Pennsylvanian  beds  here  are  lying  immediately  above  the  Missis- 
sippian  at  the  extreme  limit  of  the  Pennsylvanian  area. 

On  the  east  side  of  the  State,  there  is  valid  objection  to  the  correla- 
tion of  the  Grape  Creek  and  Danville  coals  as  Nos.  6  and  7.  According 
to  Noe,  the  shales  associated  with  these  coals  carry  plant  fossils  that 
are  not  only  distinctly  later  than  those  of  the  McLeansboro  of  southern 
Illinois,  hut  later  than  any  yet  found  elsewhere  in  the  State.4  On  the 
other  hand,  beds  outcropping  to  the  east  along  Wabash  River,  at  Cov- 
ington, Indiana,  show  a  lithologic  sequence  and  a  fauna  strongly  sug- 
gestive of  the  typical  marginal  development  of  the  Herrin  coal  zone. 
Stratigraphic  studies  carried  on  independently  by  L.  A.  Mylius,  formerly 
of  the  Illinois  Geological  Survey,  led  him  to  the  conclusion  that  the 
Grape  Creek  bed  lies  about  300  feet  above  the  horizon  of  the  Herrin  coal.5 

REVISIONS  IX  CORRELATION 

From  the  foregoing  notes  of  apparent  discrepancies,  which  are 
merely  typical  of  what  has  been  encountered  in  all  parts  of  the  Illinois 
field,  it  appears  that  some  reconsideration  of  the  correlation  is  justified. 
It  should  be  clearly  understood,  however,  that  careful  study  of  evidence 
not  yet  available  may  lead  to  conclusions  which  do  not  affect  any  major 
features  of  the  present  correlation,  hut  may  be  simply  a  modification  of 
the  now  accepted  relations.  In  any  event,  and  this  is  the  important 
point,  the  revised   correlation   will   be   based   upon   certain   hitherto   mi- 


4  Personal  communication,   1924. 
■"  Personal   communication. 


8  CORRELATION     OF    ILLINOIS    COALS 

recognized  features  of  the  coal  field  and  will  of  necessity  be  more  firmly 
established  than  at  present. 

The  foregoing  paragraphs  are  not  to  be  construed  as  indicating 
that  little  is  known  of  the  stratigraphy  of  the  Pennsylvanian  of  Illinois. 
This  is  far  from  the  truth.  The  reopening  of  the  question  of  the 
adequacy  ot  the  criteria  for  correlation  is  in  itself  a  disconcerting  feature, 
but  should  not  lead  to  the  conception  that  our  knowledge  of  the  strati- 
graphy is  in  a  chaotic  state.  Field  study  of  outcrops,  exploratory  drill- 
ing, and  the  very  extensive  mining  operations  have  combined  to  produce 
a  very  large  accumulation  of  data  relative  to  the  correlation  problem. 
Casual  inspection  of  the  field  shows  that  there  are  relativelv  large  parts 
of  the  coal  basin  in  which  the  local  stratigraphic  relations  cannot  be 
questioned. 

Beginning  at  the  north,  there  is  the  La  Salle  district  for  the  I. a  Salle  I 
or  No.  2  coal,  the  Springfield-Peoria  district  for  both  No.  5  and  No.  6 
coals,  the  Mount  Olive  and  Staunton  district,  which  i>  really  a  north 
extension  for  the  Standard  district  for  the  Belleville  coal,  and  the  south- 
ern Illinois  district  for  No.  6  and  locally  for  No.  5  coal,  and  lastly,  the 
Vermilion  County  district  for  the  Grape  Creek  and  Danville  beds,  com- 
monly called  the  No.  6  and  No.  7. 

It  will  be  noted  that  only  a  single  coal  zone  is  recognized  in  most 
ot  these  areas,  and  that  even  in  the  aggregate  only  a  moderate  part  of 
the  whole  Pennsylvanian  system  is  involved.  Further,  the  known  areas 
are  separated  by  rather  wide  gaps  in  which  the  evidence  for  correlation 
is  inadequate. 

It  is  obvious  that  any  statement  of  discrepancy  or  of  correlation 
must  be  made  on  an  assumption  of  identity  of  certain  strata  of  the  gen- 
eral section  throughout  the  State,  or  at  least  in  the  areas  in  which  corre- 
lation is  attempted.  This  is  in  itself  one  of  the  important  points  in  ques- 
tion. In  the  main,  state-wide  correlation  of  the  whole  section  has  been 
on  the  assumed  recognition  of  two  prominent  coal  beds,  the  Murphvs- 
boro  and  the  Herrin.  The  Murphysboro  bed  has  been  identified  pri- 
marily  bv  fossil  flora  in  the  overlying  shale,  and  the  Herrin  coal  by  its 
"blue-band"  or  shale  parting  and  by  its  cap  rock  of  fusulina-hearing 
limestone.  It  will  be  well  to  scrutinize  these  criteria  carefully. 

The  very  careful  studies  carried  on  in  many  parts  of  the  Illinois 
basin  by  Dr.  A.  C.  Noe  during  the  past  few  years  have  served  to  show 
that  the  similarities  between  the  flora  over  the  supposed  No.  2  in  differ- 
ent parts  of  the  State  are  not  as  marked  as  had  been  thought,  and  that 
there  is  reason  to  doubt  that  the  beds  so  correlated  are  in  all  cases  con- 
temporaneous.'''    Indeed  the  work  on   plants  from   other  horizons,  both 

6  Personal  communication,   1925. 


CORRELATION    OF    ILLINOIS    COALS  9 

ligher  and  lower  than  that  of  the  Murphysboro  coal,  has  fairly  deraoii- 
•trated  that  much  more  extensive  collecting  and  additional  careful  study 
ire  needed  before  an  adequate  paleobotanical  ladder  can  be  made.  The 
loral  zones  so  far  recognized  are  roughly  placed  in  three  distinct  strati- 
jraphic  groups  by  Dr.  Noe,  and  it  seems  probable  their  relations  are 
inderstood.  Within  the  groups,  however,  the  relations  are  not  as  yet 
letermined. 

Similar  studies  on  the  beds  near  the  horizon  of  No.  6  coal  have 
shown  that  it  is  not  safe  to  correlate  this  coal  on  the  basis  heretofore 
Itilized,  i.  e.,  the  median  shale  parting  of  the  coal  itself,  and  the  fusulina, 
j'trtyi/ia  ventricosa,  in  the  cap  limestone.  Specificallv  diamond  drill 
;ores  from  Madison  County  reveal  coal  beds  other  than  No.  6  carrying 
i  shale  parting  in  places  and  overlain  by  thin  limestone  carrying  fusu- 
inas  indistinguishable  on  present  data  from   Girtyina  ventricosa. 

The  recent  finding  of  fusulinas  in  several  distinct  beds  which  appear 
o  be  widely  separated  stratigraphicallv  has  led  to  a  recognition  of  the 
leed  of  a  critical  study  of  the  means  of  identification  of  this  interesting 
;hell.  The  work  of  Walter  Searight  makes  it  appear  probable  that 
several  types  of  fusulinas  can  be  identified  in  the  Illinois  section,  and 
lere,  as  in  the  case  of  paleobotanical  work,  the  investigations  give  prom- 
be  of  great  value  in  the  determination  of  stratigraphic  relations  within 
he  Pennsylvanian  of  the  Illinois  field. 

Other  fauna!  evidence  may  not  be  lacking  for  correlation  purposes. 
Hie  strata  are  usually  fossiliferous,  and  frequently  abundantly  so.  It 
s  unreasonable  to  suppose  that  there  were  no  distinctive  faunal  changes 
luring  the  whole  Pennsvlvanian  time,  and  so  we  may  confidently  look  to 
laleontologv  for  important  contributions  to  the  problem.  Some  results 
)f  this  nature  have  recently  been  published.7  The  point  of  difficulty, 
ust  as  in  the  case  of  plants,  is  that  as  vet  sufficient  work  has  not  been 
"eported,  either  in  collecting  or  in  identification,  to  make  possible  the 
iccurate    correlation    of    Pennsylvanian    beds    on    that    basis    alone. 

The  careful  student  cannot  but  conclude,  in  view  of  this  situation, 
hat  at  the  present  time  the  commonly  accepted  bases  for  correlation 
)f  the  Illinois  section  are  fallible  and  must  be  used  with  great  caution 

f  at  all. 


'  Savage,  T.  E.,  Marine  invertebrate  fossils  as  horizon   markers  in  the  Pennsylvanian 
OCks   of   Illinois:      Jour,    of   Oeol.,    vol.    32,    pp.    575-582,    1!)24. 


10  CORRELATION'    OF    ILLINOIS    COALS 


GEX  E  R.\  LI  ZATIONS  R  E(  WARDING  PENNSYLVA  X I A  N 
STRATIGRAPHY   AND   CONDITIONS  OF 
SEDIMENTATION 

Present  data  are  insufficient  to  permit  any  revision  of  the  existing 
correlation,  but  there  has  been  recent  accumulation  of  evidence  pointing 
definitely  toward  certain  generalizations  which  may  well  be  considered. 
It  should  be  emphasized  here  that  the  following  statements  are  by  no 
means  considered  final,  but  are  offered  in  the  hope  that  they  will  provl 
helpful  in  future  work  by  suggesting  possible  relations  which  may  bl 
upheld  or  discarded  as  new  and  better  data  appear. 

In  its  larger  features  the  Pennsylvanian  of  the  Illinois  field  is  per- 
haps to  he  considered  as  a  filling  of  a  periodically  subsiding  basin. 
Making  use  of  the  present  names  for  the  subdivisions,  we  may  say  that 
the  early  Pottsville  appears  to  he  confined  to  that  part  of  the  State  lying 
south  of  Sangamon  County.  Here  are  the  well-consolidated  shales  and 
sandstones  such  as  have  not  been  found  in  the  northern  part.  Included 
with  these  beds,  and  perhaps  typical  of  them,  are  the  "gun-metal"* 
shales  which  outcrop  in  Gallatin  and  Hardin  counties  and  have  been  re- 
ported  in   Lawrence  and   Clark  counties. 

Later  Pottsville  and  early  Carbondale  strata,  including  the  recog- 
nized horizons  of  No.  1  and  No.  2  coals,  with  some  beds  formerly  called 
No.  3  or  Xo.  4.  probably  cover  a  somewhat  wider  area.  They  appear 
to  be  rather  well  distributed  over  the  State,  reaching  from  the  southerr 
boundary  northward  to  Illinois  River  and  westward  into  Knox  Count} 
at  the  north  and  Jersev  County  at  the  south.  On  the  east  side  relation? 
are  not  clear,  but  there  is  some  evidence  that  beds  of  this  age  underlie 
the  later  deposits  along  some  of  the  margin. 

The  later  Carbondale  and  basal  McLeansboro  beds,  including  hori 
zons  of  No.  5,  Xo.  6  and  perhaps  No.  7  coals,  appear  to  have  been  mor 
widespread  originally  than  any  of  the  earlier  deposits.  They  seem  t< 
overlap  the  earlier  Pennsylvanian  locally  along  much  of  the  wester- 
margin  of  the  basin. 

A  still  later  group  of  beds,  the  uppermost  McLeansboro,  includin 
all  beds  above  the  horizon  of  No.  7  coal,  represent  the  last  Pennsylvania 
deposition.  Although  some  coal  was  deposited  at  this  time,  the  onl 
commercially  important  beds  are  found  in  the  Danville  district,  and  th 
main  deposition  was  of  clastic  and  marine  calcareous  types.  The  pron 
inence  of  the  limestone  beds  in  this  portion  of  the  section  is  an  inte 
estine  feature.      Considered   in   connection  with  the  successive  overla] 


BA  term  applied  by  early  workers  in  southern  Illinois  to  hard,  shiny,  dark  gray    sha: 


CORRELATION    OF    ILLINOIS    COALS  11 

during  the  Pennsylvanian,  the  widespread  marine  deposition  suggests 
that  in  late  Pennsylvanian  the  gap  separating  the  Iowan  and  Illinoian 
fields  was  bridged. 

So  much  for  the  general  relations  between  the  larger  stratigraphic 
units  of  the  Pennsylvanian.  The  attempt  to  distinguish  and  to  correlate 
the  smaller  units  and  individual  members  of  the  formations  involves  the 
identification  and  recognition  of  key  beds.  The  selection  of  a  datum 
horizon  is  of  the  greatest  importance  since  the  problem  involves  consid- 
eration of  separated  areas  in  which  the  geologic  columns  have  little  in 
common  except  their  lithlogic  monotony.  The  basal  stratum  of  what 
is  now  called  the  McLeansboro  formation,  the  cap  rock  of  No.  6  coal,  is 
a  stratigraphic  unit  which  gives  promise  of  rather  extensive  areal  dis- 
tribution. As  will  be  pointed  out  later,  greater  reliance  for  purposes  oi 
correlation  can  be  placed  on  the  zone  of  beds  including  this  basal  lime- 
stone than  on  the  single  stratum.  It  is  commercially  the  most  important 
bed  in  the  Illinois  section  and  is  accessible  in  hundreds  of  mines  in  a 
score  of  counties.  Another  feature  is  that  this  limestone  is  near  the 
middle  of  a  thick  Pennsylvanian  column,  the  relations  to  which  can  be 
determined  readily  for  limited  areas.  Still  another  point  for  the  selection 
of  this  stratum  is  that  if  any  changes  were  to  be  made  in  the  correlation. 
the  retention  of  the  old  number  for  this  coal  would  greatly  reduce,  in 
the  industrial  world,  the  disturbance  caused  by  a  shift  in  names  of  the 
coals. 

The  general  conditions  of  sedimentation  as  revealed  in  the  character 
of  the  deposits  are  of  great  importance  in  the  problem  of  correlation.  In 
the  Illinois  field  these  can  be  fairly  well  outlined  for  the  larger  features. 
The  marine  limestone  became  increasingly  prominent  during  Pennsyl- 
vanian time,  but  its  formation  began  in  early  Pottsville  if  not  in  the 
earliest  Pottsville.  Limestones  referable  to  this  time  are  found  in  the 
central  portion  of  the  field  as  at  Assumption,  Christian  County,  and  in 
the  marginal  portions  as  at  Alton  and  Frederick,  Schuyler  County.  It  is 
:  probable  that  further  study  of  the  sections  in  the  southern  Pottsville  belt 
will  bring  out  additional  occurrences.  Limestone  of  marine  oriein  is 
more  common  in  later  Carbondale  and  seems  to  have  reached  its  climax- 
in  late  McLeansboro.  Clastic  deposition  preceded  and  followed  that  of 
limestone,  and  although  in  some  parts  of  the  field,  as  at  Morris,  the 
hthology  suggests  continental  origin,  elsewhere  the  origin  is  perhaps 
largely  marine. 

The  deposition  of  coal  was  apparently  restricted  to  small  separated 
-wamps  in  Pottsville  time,  but  these  areas  were  scattered  over  much 
of  the  southern  half  of  the  State.  In  Carbondale  time  the  relief  of  the 
basin  was  probably  less  and  the  swamps  of  coal  deposition  were  proba- 


12  CORRELATION    OF   ILLINOIS    COALS 

My  correspondingly  greater  in  size.  It  is  not  likely,  however,  that  the 
separate  swamps  were  of  great  extent.  Detailed  studies  in  the  La  Salle 
and  Murphysboro  fields  indicate  that  the  thicker  portions  of  the  coal  beds 
grade  laterally  in  short  distances  to  carbonaceous  shales  or  to  splits  ii 
clastic  beds.  In  late  Carbondale  and  earlv  McLeansboro  time,  emialh 
widespread  coal  swamps  appear  to  have  existed,  but  here  again,  although 
the  area  of  thicker  coal  is  much  greater  than  in  the  preceding  epoch,  the 
main  portion  of  the  held  seems  to  have  been  the  site  of  elastic  deposi 
tion,  for  the  coal  beds  <^rade  laterally  into  such  beds  relatively  near  tin 
main  mining  districts. 

An  important  exception  is  probably  to  be  found  in  the  area  near 
the  Duquoin  fold.  Here,  during  the  later  Carbondale,  conditions  appeal 
to  have  been  exceptionally  favorable  to  coal  deposition,  and  in  the  deepei 
part  of  the  downfold  north  of  Franklin  County  we  may  confidently  lool 
for  extensions  of  the  thicker  portions  of  the  llerrin  coal.  To  the  east 
or  west,  farther  north,  however,  it  is  likely  that  the  coal  swamps  wen 
limited  by  land  or  by  the  open  sea.  In  either  case  the  formation  o 
commercially  important  bodies  of  coal  was  impossible.  This  epoch  wa- 
followed  by  one  in  which  coal  swamps  were  of  little  importance.  Thii 
coal  beds  demonstrate  the  presence  of  swamps  which  may  have  beei 
widespread  in  the  nearly  flat  basin  of  late  McLeansboro  time.  but.  if  so: 
they  were  short  lived  and  did  not  produce  any  important  accumulation 
of  vegetal  matter,  except  on  the  east  margin  where  in  limited  zones  wen 
deposited  the  Grape  Creek  and   Danville  beds. 

Study  of  the  stratigraphic  details  of  the  Illinois  sections  emphasize 
two  points.  (1  )  Although  several  rather  extensive  areas  appear  to  hav< 
been  swamps  at  different  times,  there  are  few  sections  which  show  mor 
than  one  important  coal  bed.  (2)  A  single  coal  bed  in  one  area  ma- 
be  represented  in  adjacent  territory  by  several  beds  separated  by  clastic- 
For  correlation  purposes,  it  is  probably  necessary  to  consider  these  zone 
of  coal  rather  than  the  individual  beds  as  stratigraphic  units.  In  sue' 
zones  may  be  combined  separate  beds  of  approximate  contemporaneiti 
without  assumption  of  definite  relations  between  them. 

From  the  general  statements  of  depositional  conditions  during  tin 
period,  it  is  clear  that  the  most  widespread  recognizable  beds  are  th 
marine  limestones.  The  close  similarity  of  all  the  beds  in  section 
stratigraphicallv  adjacent  makes  it  impossible  to  correlate  separate  roc 
masses  on  the  basis  of  limestones  alone.  By  taking  into  consideratio 
the  conditions  of  sedimentation  as  revealed  by  the  lithohgic  sequenci 
which  include  the  limestones,  it  has  been  possible  to  make  satisfactor 
correlation  over  short  distances.  There  seems  to  be  no  obstacle  t 
o-reater  extension  of  this  mode  of  correlation.     By  this  is  not  meant  an 


CORRELATION    OF    ILLINOIS    COALS  13 

[eparture  from  standard  geologic  practice.  The  careful  observer  will 
lot  fail  to  note  that  correlation  by  fossils  alone,  either  plant  or  animal 
orms.  is  only  feasible  after  the  correlation  ladder  has  been  prepared 
rom  a  study  of  the  fossils  in  a  series  of  beds  whose  relations  are  known. 
Phis  has  not  as  yet  been  done,  nor  will  the  mere  presence  of  distinguish- 
ible  forms  serve  in  this  instance  to  prove  a  correlation.  Not  only  the 
listinctions  but  also  the  relationships  between  the  forms  recognized  as 
ypical  of  the  several  stratigraphic  zones  must  be  determined  before  the 
evidence  can  be  accepted  as  final.  It  is  probable  that  the  final  and 
:orrect  correlation  will  be  accomplished  only  by  the  active  cooperation 
if  workers   in   all   three   lines. 


lER'S" 

IBRARY  BINDERS 
507  3.  Goodwin 
Urbane.  N. 


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